The sun is the largest source on Earth of non-anthropogenic noise in the hundreds of megahertz, MHz, and the gigahertz, GHz, range. The usual measure of solar noise is in terms of solar flux units or sfu, where 1 sfu equals 10−22 watts per meter squared hertz or W/(m2 Hz).
Solar radio bursts have been studied for many years, and they are believed to be created by gyrosynchrotron emission by energetic electrons in the sun's magnetic field. Data collected during the 20th solar cycle in 1966–1978 suggests that large events occur perhaps a few times in a solar cycle, see, e.g., W. R. Barron et al., “Solar Radio Emission,” Handbook of Geophysics and the Space Environment, A. S. Jursa, ed., Chapter 11, AFGL, USAF, 1985. Events of lesser magnitude are more numerous. A histogram of the distribution of the amplitude of events has been measured for the 20th solar cycle, see, e.g., the above-referenced W. R. Barron et al., “Solar Radio Emission,” but there is no reason to believe that the distribution is typical of other solar cycles. There is some evidence to indicate that other cycles, such as the 18th when solar radio noise was discovered through its interference with radar in the Second World War, may have been worse. Statistics do not exist on the polarization of these bursts, and the changes with time of the polarization, if any.
Embedded within the radio bursts are often burst “spikes,” see, e.g., A. O. Benz, “Millisecond Radio Spikes,” Solar Physics, 104, 99–110, 1986; and H. Isliker et al., “Catalogue of 1–3 GHz Solar Flare Radio Emission,” Astronomy and Astrophysics, 104, 145–160, 1994. Such spikes can reach amplitudes that are several orders of magnitude more intense than the broadband microwave continuum, see, e.g., the above-referenced H. Isliker et al., “Catalogue of 1–3 GHz Solar Flare Radio Emission.” Spike bursts have durations of tens to hundreds of milliseconds, although detailed statistics do not yet exist. The physical processes producing the spikes are uncertain. One proposal, based on the intensity of the burst, is a maser-type process. The center frequencies of the spike bursts vary from spike to spike, and from large burst event to event. The bandwidth of the bursts is variable, ranging from a fraction of a percent to a few percent of the central frequency, see, e.g., A. Csillaghy et al., “The Bandwidth of Millisecond Radio Spikes in Solar Flares,” Astronomy and Astrophysics, 274, 487–496, 1993. The bandwidth near a gigahertz may be, for example, a few megahertz to tens of megahertz.
However, while attempts have been made to study these solar bursts and embedded spikes over the years, numerous aspects relating to how they may affect various systems on earth and in space are not well characterized. This is the case with respect to wireless communication systems and how such a noise source may adversely affect communications in wireless systems.